There are 390,000 gay marriages in the U.S. The Supreme Court could quickly make it half a million.

Craig Hayes of Madison, Wis., waits in line to win a coveted seat in the gallery to watch arguments in the same-sex marriage case Obergefell v. Hodges, at the U.S. Supreme Court in Washington Monday. (REUTERS/Jonathan Ernst)

The U.S. Supreme Court will begin to hear oral arguments Tuesday on whether the Constitution forbids states from banning marriage between same-sex couples. If the court decides in favor of same-sex marriage, we could very well see a continued dramatic increase in the number of married same-sex couples in the United States.

Today, there are about 390,000 married same-sex couples in the country, according to Gallup. There are also 1.2 million adults living in a same-sex domestic partnership. So about four in 10 of all same-sex couples in the United States are married.

The Gallup data suggests same-sex marriage has increased dramatically in the past two years. Here's what that figure looks like, charted by combining the Gallup numbers and Census data from the American Community Survey between 2010 and 2013 (2014 data is not yet available). It is important to note these two sources used different methods to obtain their data, and measuring the incidence of very small population characteristics is difficult, even with large samples sizes. So the trend line is somewhat flimsy.

The most obvious reason same-sex marriages are increasing is because the number of states where it's legal have increased. By the end of 2010, same-sex marriage was legal in just five states. By the end of 2013, it was legal in 16. And the steep jump between the Census's 2013 numbers to Gallup's numbers today coincides with the number of states where it was legal more than doubling in 2014 alone.

Other factors that could contribute to the rise are the increased acceptance of same-sex marriage, which could make more couples feel comfortable marrying -- as well as the different ways of tracking the data.

So how many same-sex couples could marry if the Supreme Court made it legal nationwide? Using the 2013 Census, which breaks numbers down by state, there were 183,252 same-sex households in states where marriage isn't yet legal. It isn't a perfectly accurate number (some couples in these states report being married even though it's not legal, for example), but it suggests the number of married same-sex couples could grow substantially with a court ruling favorable to same-sex marriage.

In states where it is legal, about half of same-sex couples are married, and about half are in domestic partnerships. Those numbers suggest nationwide gay marriage legalization, in addition to marriages that will continue in states where it's already legal, could quickly push the number of same-sex marriages from 390,000 to closer to half a million.

The nation's highest court is hearing Obergefell v. Hodges, a case that examines if same-sex couples have a constitutional right to marry. Here's what you need to know about the case that could make gay marriage legal across the nation. (Julie Percha/The Washington Post)